Generally, an exhaust system of a vehicle is either a close-coupled catalytic converter (CCC) type or an under-floor catalytic converter (UCC) type according to disposition of a catalytic converter, considering engine compartment layout and the degree of exhaust gas reduction demand. In the CCC type, the catalytic converter is disposed close to each runner of an exhaust manifold, and in the UCC type, the catalytic converter is disposed relatively far from an exhaust manifold so that a demand for making exhaust gas flow uniform is relatively small.
There have been various directions in research and development for improving exhaust gas characteristics in such exhaust systems. Factors for improving exhaust gas characteristics can be summarized as follows: an improvement of flow uniformity upstream of a catalytic converter; a selection of a mounting position of an oxygen sensor; an improvement of a catalytic activating temperature during cold start; and a suppression of an increase of back pressure.
In the UCC exhaust system that is usually adopted for a V-type engine, first and second exhaust pipes are respectively coupled to exhaust manifolds through front catalytic converters. The first and second front exhaust pipes are joined together at a coupling portion.
However, in such a conventional exhaust system for a V-type engine, the first and second front exhaust pipes are typically perpendicularly coupled at the coupling portion, so flow of exhaust gas inside the coupling portion becomes irregularly turbulent. Furthermore, the sectional area of the exhaust line in the coupling portion substantially decreases so that a large pressure increase occurs in that portion, and this may cause an increase of back pressure, so that full load performance becomes deteriorated. Because the distance between the exhaust manifolds and the main catalytic converter 115 is large, exhaust gas temperature in the main catalytic converter may become lower than a catalyst activating temperature.
The information disclosed in this Background of the Invention section is only for enhancement of understanding of the background of the invention, and should not be taken as an acknowledgement or any form of suggestion that this information forms the prior art that is already known to a person skilled in the art.